Data recorders for collecting and managing vehicle operational data or messages are known in the automotive and heavy duty truck industries. Data recorders are particularly useful in motor vehicles forming fleets such as taxis, buses and sales/service vehicles as well as product delivery/shipping vehicles including medium and heavy duty trucks to name a few. Vehicle operating data collected and managed by such data recorders are typically used by the fleet owner/manager for vehicle diagnostic purposes and/or for analyzing driver performance over one or more trips.
Since data analysis typically takes place some time after it is recorded, it is often beneficial, and sometimes necessary, to have information relating to the date and/or time at which the data was recorded. Data analyzers having such capability are thus able to link the vehicle operational data to vehicle operating conditions existing during data recordation such as, for example, a particular driver or group of drivers, weather conditions, geographic locations, road grade conditions encountered, and other vehicle operating conditions. To this end, designers of electronic data recorders have included therein real-time clocks which are operable to "time-stamp" the vehicle operational data or messages as they are recorded. As used herein, the term "time stamp" is defined as a real-time data identifier that forms a portion of data record for any given data message, and that identifies a calendar date, and optionally a time of day, at which the particular data message was recorded.
While such known real-time clock circuits or subsystems have been successful in accurately implementing data time stamping techniques, they have at least one inherent drawback associated therewith. Specifically, such circuits or subsystems rely on continuous electrical power provided thereto for accurate operation, and such accurate operation ceases when power is lost due to tampering, maintenance and/or other factors. When power is restored, the time value of the real-time clock circuit or subsystem is typically reset to some default value other than the actual calendar date and time of day, and therefore provides for inaccurate time stamping of subsequently stored data.
To combat the foregoing problem, designers of data recorders incorporating a real-time date/time stamping feature typically provide for some type of back up power source to supply electrical power to the real-time clock circuit for subsystem in the event of power loss from the primary power source. Examples of vehicle/engine data recorders incorporating back up, or stand-by, battery operation in the even of primary power loss are described in U.S. Pat. No. 4,303,850 to Juhasz et al. and U.S. Pat. No. 5,191,529 to Ramsey et al.
While such back up battery systems solve the above-described problem associated with the loss of power to the real-time clock circuit or subsystem, they introduce new drawbacks and concerns. For example, back up power systems are expensive to implement, both monetarily and in terms of space consumption. As more engine and vehicle functions become subject to electronic control, available circuit/system space within the vehicle, and specifically within a typical engine/vehicle control computer, correspondingly diminishes. Consumption of precious circuit/system real estate by a seldom used back up power system is therefore difficult to justify. Moreover, such back up power systems are not fail safe, and it is typically not until primary power failure occurs that the failure of the back up power system is discovered. Further, erroneous resetting of typical real-time clock circuits or subsystems can result from many factors or conditions other than primary power loss. In such cases, any back up power system is unable to prevent resetting of the real-time clock value to its default time value, and the actual clock value is resultantly lost.
What is therefore needed is a system for providing accurately time stamped vehicle operational messages which overcomes the problems associated with temporary loss of electrical power or other conditions which cause resetting of the real-time clock or subsystem, and which does not suffer from any of the drawbacks associated with the use of back up, or stand-by, power systems. Such a system should ideally be inexpensive and easily implemented while providing for highly accurate time stamped vehicle operational messages.